Sahih Muslim Book 4, Hadith Number 1226.

Chapter : Desirability of seeking refuge from the torment of the grave.

Thauban reported: When the Messenger of Allah (may peace be upon him) finished his prayer, he begged forgiveness three times and said: O Allah ! Thou art Peace, and peace comes from Thee; Blessed art Thou, O Possessor of Glory and Honour. Walid reported: I said to Auza’i: How Is the seeking of forgiveness? He replied: You should say: I beg forgiveness from Allah, I beg forgiveness from Allah.”